1. Field of the Invention
The present invention relates to the use of an adhesive based on crosslinkable silicone compositions for the temporary adhesive bonding of two chemically similar substrates. The term “temporary adhesive bonding” means that the substrates can be parted from one another again at a desired point in time. When the bond is parted, the adhesive layer remains almost completely on one of the two substrates, and so very little or no material of the adhesive layer requires removal by mechanical or chemical methods from the other substrate subsequently.
2. Description of the Related Art
In industrial operations, one requirement is for temporary adhesives between two chemically similar substrates that are to part specifically again from the desired substrate after use. Preference here is given to adhesive fracture between a substrate and the adhesive layer. Formulations used to date are capable of bonding two similar substrates, but without preferring one substrate. In accordance with the prior art, it has been possible so far to deliberately adjust the adhesive forces, by means of physical or chemical methods, only by way of pretreatment of the respective substrate. Examples of physical methods are abrading of the surface, UV irradiation, flame treatment, corona treatment, or plasma treatment. With pretreatment steps of these kinds, the surface or the near-surface layer of the substrate material is activated—that is, functional groups, primarily polar groups, are formed. Examples of chemical methods are pretreatment with adhesion-promoting additives (also called primers) or pretreatment with substances which prevent adhesion (also called release agents).
The market requirements from the fabricators of semiconductor components with regard to the chips to be fabricated, in particular, call for ever lower overall heights. One option for reducing the overall height of the shaped component is to make the wafers thinner. The grinding operation may be carried out before or after shape singularizing (dicing).
As a result of the wafers becoming increasingly large and increasingly thin, the structural strength of the wafer is reduced. As a result of this, the large, thin wafers may break when being worked on with the devices and materials that are commonly used, such as the dicing tapes that are nowadays used, for example, owing to insufficient mechanical support. Moreover, the wafer structures which protrude by up to 100 μm or more (bumps) may not be fully enclosed, without any voids, by the adhesive films (tapes) that are presently used, and so the voids which remain may lead in general, during vacuum working, to destruction or impairment of the thin wafer.
One possible technical solution to this problem is to bond the wafer to a hard substrate (for example, to another wafer or to a wafer support such as glass, for example) via a temporary adhesive layer, in order to increase the mechanical strength, and then to carry out the desired working operations, and finally to detach the wafer, which at this point has a thickness of only 10-100 μm, from the substrate again. The substrate attached via the adhesive serves here as a mechanical reinforcement during the grinding and downstream working operations.
Later working on the thinned wafer also includes the formation of resistant structures by plasma etching and operations such as, for example, metal plating and residue cleaning.
One possibility for fixing the semiconductor wafer on a support is represented by adhesive tapes. EP 0838086 B1 describes a tape composed of a thermoplastic elastomer block copolymer for use for the processing of semiconductor wafers. The limited elasticity of the material, however, leads to problems when wafers having surface structures (bumped wafers) are used. The thermoplastic properties of the material, moreover, lead to reduced heat stability, such stability being necessary for the back-side operations (plasma processes, CVD, etc.) that follow the thinning of the wafer (back-side grinding).
WO 2009/003029 A2 claims thermoplastic organic polymers (imides, amideimides, and amideimide-siloxanes) for use as a temporary adhesive between a wafer and a carrier.
WO 2009/094558 A2 describes the temporary adhesive bonding of a wafer and a carrier, the adhesive bond taking place not over the entire area but instead only in the edge region. Following a grinding operation and possible back-side operations, the adhesive bond is destroyed chemically, photochemically, thermally, or thermomechanically.
EP 0603514 A2 describes a method for thinning a semiconductor wafer, where the adhesive material used is suitable at most for temperatures of 200° C.
The US application US 2004/0121618 A1 describes a liquid adhesive which is suitable for spincoating operations and which consists of a thermoplastic polyurethane, and also dimethylacetamides and propylene glycol monomethyl ether as principal components.
EP 1089326 B1 claims a support for wafers that consists of a silicone elastomer covered with a dustproof film, the release force between the film and the silicone layer being from 5 to 500 g/25 mm (according to JIS K 6854). Also described is a method for producing a wafer support.
WO 2004/051708 A2 describes a method for the working of a wafer in which a release layer is applied between the front side of the wafer and the support layer (adhesive). The production of this release layer represents an additional step in the production chain, leading to prolonged working times and to higher production costs.
The major problem is that similar materials, such as, in this case, the previously processed silicon wafer, which on its surface may also contain materials other than silicon, is on the one hand to be joined firmly to a carrier, which likewise consists of silicon, using an adhesive layer, while on the other hand, after the steps that are customary in wafer processing, such as wafer grinding or thinning, coating, etc., for example, they are to be separated extremely simply and selectively at the wafer surface.
The systems used to date in accordance with the prior art are unsatisfactory for certain processing techniques such as, for example, the thermal deposition of silicon dioxide or high-temperature dielectrics, since the high-temperature stability of the polymers used is inadequate. In addition to the necessary temperature stability of up to more than 300° C., moreover, the correct balance and optimization of the formulation in respect of the mechanical parameters (during grinding of the wafer back), the chemical stability with respect to various chemicals customary in chip processing, the minimal release of volatile secondary products (risk of contamination), and the viscosity of the uncured adhesive (during application to the wafer) are important objectives in the development of a suitable material in the sandwich system under consideration.
Particularly within the semiconductor industry, therefore, there is a need for a selectively adhering adhesive for temporary adhesive bonding of two chemically similar substrates. The adhesive must be suitable for application by an industrially practicable process (for example, spray coating, printing, dipping, rotational coating), and at the desired point in time must be able to be parted from the desired substrate without residue by way of suitable methods. Furthermore, besides the supporting bond, e.g. a hard bond to the substrate, the adhesive is also to have certain elastic properties.
These requirements apply in particular for wafer bonding processes. The properties of the adhesive are to allow the processing of the wafer without breakage or damage to the latter, and are also to be able, furthermore, to enclose the front structures of the wafer in a supporting manner.